This invention relates to colored polyester plastics produced through the introduction of coloring agents after initial polymerization has finished but before completion of a solid stating step. As a result, the plastic is melt-extruded and mixed with the coloring agent and subsequently subjected to solid stating wherein the base polymer is accorded an increase in molecular weight and the coloring agent becomes integrated therein. Such a method permits avoidance of introduction of a coloring agent to the polymerization reaction chamber that results in extensive staining (particularly upon a change of desired colors for the next batch of plastic). Such a method also permits avoidance of the introduction of a coloring agent to an injection molding process that results in uneven color dispersion and sub-optimal physical properties within the ultimate article. The inventive procedure produces colored thermoplastics of substantially the same shades and uniformity as with the aforementioned precoloring or masterbatch process.
Polyesters, most notably polyethylene terephthalates (PET), have been utilized for a variety of purposes, including the production of fibers, films, plastic articles, and engineering resins. In particular, PET is present in different types of resin, such as melt-phase resin (a.k.a., reactor-grade polyester), which is used to produce polyester fibers, and solid-state resin, which is used as the primary source of plastic soda bottles and other polyester articles. The differentiation between such types of resins exists in the molecular weight, reported as intrinsic viscosity (IV), of the particular resins. The greater the IV of the polymer, the greater the tensile properties of the polymer itself. In order to increase molecular weight (as measured by IV) of bottle-grade resins, solid state polymerization must be undertaken in addition to the standard polymerization steps followed to produce the polyester itself. Solid stating generally consists of exposing the already-polymerized polyester resin to relatively high temperatures to form longer chain polymers. The resultant IV of the bottle-grade resin must lie in the range of between about 0.72 and about 0.85 (wherein fiber-grade resin possesses an IV of at most about 0.6).
Colored Polyester resins and thermoplastics have been utilized in a myriad of applications requiring a high IV, including, without limitation, containers, bottles, casings, and the like. Such colorations provide aesthetics and permit identification of certain products and articles. Numerous types of coloring agents have been utilized for such purposes, including, again, without limitation, pigments, dyestuffs, polymeric colorants, and the like. Such varied and different coloring agents are generally introduced within such polyesters in similar ways.
Traditionally, and most prominently, coloring agents have been added during a precoloring or masterbatch process in order to produce colored polyester pellets. Such pellets generally include other additives, such as antistatic agents, plasticizers, and the like, and, since they include the aforementioned coloring agents, are then introduced to the standard extrusion screw (or other type of) melting mechanism. From there, the molten plastic can then be extruded, injected, etc., and molded into the target article. Because of the thorough mixing during the melting phase, such ultimate articles will most likely exhibit substantially uniform colorations throughout and optimum IV properties within the final article.
However, problems do exist with such a traditional method of coloring polyester thermoplastic and resinous articles. Most notably, the polymerization reaction vessel (which, on an industrial scale, is generally measured by volume in the thousands of gallons), in which the entire plastic, coloring agent, and other additives, are initially mixed together, is subject to staining by the utilized coloring agents. The resultant stains must be cleaned, particularly when the next plastic to be polymerized must be of a different color. Such required cleaning potentially adds appreciable costs to colored polyester production and certainly can cost valuable time during such production.
Alternative coloring methods have been followed to alleviate such problems, however they have proven to be less desirable due to difficulties in producing uniform colorations within the target plastic articles. For instance, within injection molding processes, the coloring agents can be introduced during the actual injection molding step. While this process is acceptable for low color loading applications, it, unfortunately, also presents a number of problems especially with high color loading applications. Most notably, the coloring agent potentially functions to deleteriously reduce the IV of the polymer melt. Such modifications of the IV of the polymer results in difficulty in molding and produces final articles with sub-optimized physical properties such as lower or uneven burst strength, poor color dispersion, and more pronounced stress cracking throughout. Furthermore, the introduction of coloring agent at the injection molder, especially at high color loadings, has resulted in processing difficulties, including extended cycle times, screw slippages, and inconsistent injection pressures.
As such, other alternatives are necessary, both to avoid such difficult injection molding coloring procedures (which result in uniformity and IV problems) and the highly staining, and thus costly traditional precoloring or masterbatch processes. Any alternative methods must not deleteriously affect the IV of the resin itself. To date, no such viable alternatives have been developed or followed for polyester article production, particularly those which provide a solid-stated resin with an acceptable IV range for utilization within the desired rigid applications.
Therefore, an object of the invention is to provide a colored polyester plastic or resin article exhibiting substantially uniform colorations and optimum physical properties throughout and produced through a method which does not include any staining by the utilized coloring agent within a melt polymerization reaction vessel. Another object of the invention is to provide a colored thermoplastic resin wherein the coloring agent is introduced after the polymerization step and before the completion of the solid stating procedure, wherein the intrinsic viscosity of the colored polymer after solid-stating is similar to that of the correlative, uncolored polymer, preferably at a level between about 0.60 and 0.86, more preferably between about 0.70 and 0.86. Another object of the invention is to provide a relatively inexpensive but greatly improved method for coloring polyester thermoplastic materials.
Accordingly, this invention encompasses a method for producing colored polyester thermoplastic or resinous articles comprising the sequential steps of
a) providing a molten uncolored resin;
b) introducing said resin of step xe2x80x9caxe2x80x9d into a pre-pellet formation device, optionally with at least one coloring agent;
c) forming pellets of said resin of step xe2x80x9cbxe2x80x9d, and optionally coating said pellets with at least one coloring agent;
d) introducing the pellets of step xe2x80x9ccxe2x80x9d into a solid stating vessel, optionally with at least one coloring agent; and
e) molding the colored solid stated polyester of step xe2x80x9cdxe2x80x9d into a polyester article; wherein a coloring agent is added in at least one of steps xe2x80x9cbxe2x80x9d, xe2x80x9ccxe2x80x9d, or xe2x80x9cdxe2x80x9d.
The term pre-pellet formation device is intended to encompass any device in which the polyester is introduced after polymerization is complete, but prior to production of polyester pellets. Thus, such a device may include, without limitation, mixers, such as gear pumps, static mixers; extruders for the polymer prior to cooling and ultimate cutting into pellets, such as extrusion screws, and the like; polymer sheet producers and cutters, such as calendaring equipment, die face pelletizers, and the like; and pre- or post-mixing devices in combination with such other devices. Basically, this term thus encompasses any standard component within a polyester molding line which is oriented after the initial polymerization device and before the device which produces the polyester pellets for further processing.
The term xe2x80x9csolid stating vesselxe2x80x9d is intended to encompass any standard solid stating device for polyester production which permits an increase in molecular weight (and thus IV) through an increase in exposure temperature to a level just below the melting point of the specific type of resin involved. Such a procedure is discussed in greater detail below.